– "Het vlindereffect is de theorie die zegt dat het gefladder van een vlinder ergens anders ter wereld een tornado kan veroorzaken."("The butterfly effect is the theory which states that the fluttering of a butterfly can cause a tornado in another part of the world.")

I thought a vlindermes was a flick-knife or switchblade.
Also, it is more common to hear the phrase ‘butterflies in my tummy’ rather than stomach. This also seems to translate more literally as buik is tummy and maag is stomach.
Arme Frank – hij heeft alleen maar pech!

In English you can use butterflies in your tummy to refer to a feeling of nervousness (e.g. before an exam). Is this the same in Dutch? I was under the impression that in Dutch the expression is used more in the context of being in love.